According to the principles of electromagnetic induction, if only one current a entered the ring at A, and the direction of the winding be suitable, a negative pole (-) will be produced at A and a positive pole (+) at B, so that a magnetic needle pivoted in the center of the ring would tend to point vertically upward towards A. Now suppose that at this instant, corresponding to the beginning of an alternating current cycle, a second current b, differing in phase from the first by 90 degrees, is allowed to enter the ring at C. As shown in fig. 1,659, when the pressure of the current a is at its maximum, that of the current b is at its minimum; therefore, even a two phase current, at the beginning of the cycle, the needle will point toward A.
Fig. 1,660.—Production of rotating magnetic field in a two pole two phase motor. The poles are numbered from 1 to 4 in a clockwise direction. Phase A winding is around poles 1 and 3, and phase B winding, around poles 2 and 4. In each case the poles are wound alternately, that is, if 1 be wound clockwise, 3 will be wound counter clockwise, thus producing unlike polarity in opposite poles. Now during one cycle of the two phase current, the following changes take place, starting with pole 1 of N polarity and 3, of S polarity:
| One Cycle | ||||
|---|---|---|---|---|
| Degrees | 0° to 90° | 90° to 180° | 180° to 270° | 270° to 360° |
| Polarity | 1N - 3S | 2N - 4S | 3N - 1S | 4N - 2S |
Fig. 1,661.—Diagram showing resultant poles due to two phase current.
Fig. 1,662.—Diagram of two phase, six pole field winding. There are six coils in each phase, as shown. The coils of each phase are connected in series, adjacent coils being joined in opposite senses, thus, for each phase, first one coil is wound clockwise, and the next counter clockwise.